This book is really really good. The setting is very cool and very dark and the race and class options mesh well with it.
The new exhaustion rules are fantastic and I am using them in my Dark Sun 5e campaign, along with some of the new spells and tweaks to the spell lists.
There were a couple things this book tried to do that didn't really pan out fully, and will need further clarification, maybe in future PDF updates?
The first of these was the introduction of Blood and Grit points. Hit points essentially become grit points, and you get your max hit dice (no Con modifiers) per level. You get Blood points equal to your level plus your Strength and Con modifiers. When you take a critical hit, you lose blood points, which is really really bad. Grit points are regular damage. This all seems fine and good until you try to apply this to monsters. There were a lot of entries from the Monster Manual that just didn't mesh well with these rules. There needs to be a mention of how to calculate monster grit and blood points somewhere, because using the rules for characters just doesn't work.
The second was the rules pertaining to armor and weapons. The weapon breakage rules were taken almost whole cloth from the Dark Sun 4E campaign setting. While not necessarily bad, they have the potential to be abused (essentially giving players a minor monetary cost to reroll critical misses). Physical armor lets you absorb critical hits depending on how heavy the armor is. While thematically a cool rule, it makes no mention of other sources of armor or AC bonuses. There are class features, magic items, spells, and other rules that grant armor bonuses, not to mention natural armor but this book makes no mention of those.
So while the majority of the book is very good and well written (with beautiful art), I found these two rather large and sweeping rules changes to be a bit jarring, and lacking in playtesting perhaps?